1. Field of the Invention
This invention relates generally to apparatus using metal hydrides as a means to store and operate with hydrogen, and, more specifically, to the use of such hydrogen and in operation in which the hydrogen gas is purified during storage or operation.
2. Background Art
Inexpensive but essentially pure sources and storage of hydrogen gas (H2) is increasingly important to the production of energy as economic and environmental factors compels a shift away from dirty petrochemical fuels. Unless efforts are made to retain the purity of hydrogen gas, impurities, such as oxygen, water vapor and carbon monoxide, inevitably become entrained within a stream of hydrogen gas. These impurities impede the operation and/or efficiency of any device which stores the hydrogen gas, or which utilizes the hydrogen gas in its operation.
Purification of a hydrogen gas stream was an elaborate procedure that, in most cases, involved a net energy input into the hydrogen utilization system. For example, commonly assigned U.S. Pat. No. 5,250,368, drawn to a metal hydride battery and metal hydride hydrogen storage system, teaches a combination molecular sieve dryer and electrical resistance heating wire to remove water vapor when the hydrogen stream is directed in one direction, and by heating, re-introduces the water vapor back into the hydrogen stream when it is directed in the opposite direction. This system is capable of inhibiting entry of the water vapor into the hydrogen storage chamber, where it can cause the metal hydride hydrogen storage material to deteriorate and lose storage capacity.
Later modifications of such systems that purified hydrogen when passing in a stream from one part of a metal hydride battery system to another part included a passive hydrogen purification for hydrogen gas delivery, for example, as described in commonly assigned U.S. Pat. No. 5,688,611. In the metal hydride battery described in that patent, a formulation of a metal hydride material is dispersed within a matrix of a silica gel powder. The silica gel powder provides for absorption of water vapor before the hydrogen is absorbed in the metal hydride. The metal hydride storage medium itself may include corrosion resistant elements, and an optional surface film which is water vapor and carbon-oxide repellant. Such a film is taught in commonly assigned U.S. Pat. No. 5,532,074.
Other modifications to such metal hydride battery systems are taught in commonly assigned U.S. Pat. No. 4,781,246, drawn to a thermally reversible heat exchange unit for use in any of a number of devices utilizing the cycling of hydrogen gas in a heat transfer or hydrogen storage operations. Examples of the apparatus or systems in which heat exchange units described in U.S. Pat. No. 4,781,246 may be utilized are refrigerators, heat pumps, air conditioners, compressors and hydrogen storage devices, including hydrogen purifiers. All of these systems require an efficient method to isolate the water vapor from the metal hydride storage medium.
An automatic system is taught in commonly assigned U.S. Pat. No. 6,042,960 which inhibits transfer of water vapor in the absence of the pressurized flow of hydrogen in the context of a battery system.
Hydrogen gas streams utilized in other than metal hydride battery systems require modifications to these systems toward providing greater efficiencies. For example, in commonly assigned U.S. Pat. Nos. 5,450,721 and 5,623,987, an air conditioning system, and a modular manifold hydrogen gas delivery system are described and claimed. Those systems utilize the different hydrogen absorption characteristics of specified metal hydride alloys to provide a sudden heat energy transfer from or to a desired location. Because the system is closed, and does not introduce new hydrogen gas or other elements into the system which can affect the sorption characteristics of the metal hydrides therein, thus a filter for removing water vapor and other gas impurities was not considered necessary. However, it is now known that even in xe2x80x9chermetically sealedxe2x80x9d systems, gaseous impurities may be introduced at the initial start-up and may even enter into such a system during operations conducted at high pressures or from outgassing from the internal wall surfaces and cracks, or from diffusion through the walls.
When a repeating cycle of hydrogen absorption and desorption is used in a heat exchange cycle, impurities in the gas stream can result in the deterioration of hydriding capacity. Hydrogen absorption in a metal hydride alloy as used in heat exchange units is accompanied by a heat of formation which is exothermic. In order to continuously absorb hydrogen to an alloy""s maximum capacity, heat must be removed from the bed. The rate at which a hydride alloy can absorb or release hydrogen is dependent upon the rate at which heat can be transferred into or out of the alloy. Increasing the heat transfer rate will allow the processing of higher flow rates, or alternatively, the same flow rate can be processed by a proportionately smaller amount of alloy. Therefore, small containers capable of rapid heat transfer can handle high flow rates.
With each thermal cycle, the metal hydride alloy in a container is first filled to capacity and then emptied. Gaseous impurities can react with the hydride alloy causing a reduction in its hydrogen storage capacity and may inhibit the further absorption of hydrogen gas. The net result causes a decline in hydrogen throughput with each thermal cycle. For this reason, thermal compression of hydrogen using metal hydrides has been restricted to relatively pure hydrogen streams (99.995%) that have less than 50 ppm of active gas impurities. Although hydrogen purification systems can be used to remove impurities, the purification systems themselves are often complex, expensive to maintain, and, for hydrogen produced at atmospheric pressure, would require their own motive force in the form of a mechanical compressor or blower. These disadvantages offset benefits that could be derived from thermal compression.
The need for a filter to remove gaseous impurities from a hydrogen gas stream has been found in a variety of applications. Many of the applications in which such filters are utilizable differ in essential respects from the applications in which such filters have been utilized heretofore. For example, an application in which metal hydride combinations re utilized to compress hydrogen gas is described in commonly assigned U.S. Pat. Nos. 4,402,187 and 4,505,120. One major difference is that a hydrogen gas stream enters the compressor at an inlet and exits at an outlet at much higher pressure. The continual addition of new hydrogen into such a system introduces a continua stream of impurities that are entrained in the hydrogen. Although most hydrogen compression systems are capable of self-cleansing of a certain amount of impurities in the hydrogen gas stream, the continual addition without a purge of the impurities can overwhelm a system so that it becomes non-operational. Other applications of hydrogen gas also utilize a purification device, as will be more fully described in the detailed description below. To avoid unnecessary repetition, the description and teachings of the above mentioned commonly assigned U.S. Pat. Nos. 4,402,187; 5,450,721; 4,505,120; 4,781,246; 5,250,368; 5,532,074; 5,623,987; 5,688,611 and 6,042,960 are each incorporated herein by reference where appropriate as if fully set forth herein, for purposes of enablement of this application. A need in the operational transfer function of a hydrogen gas stream exists for removing impurities in applications beyond those heretofore known. A need is also apparent for a purification device for a hydrogen gas stream that is capable of removing not only water vapor, but also minute quantities of other types of gas impurities, such as oxygen (O2), carbon monoxide (CO) and carbon dioxide (CO2).
Accordingly, what is disclosed and claimed herein is a passive purification device that is usable within a hydrogen gas transport stream, preferably in line with a conduit, that is standard for a number of hydrogen gas storage and utilization applications.
In one embodiment, such a passive purification device in a thermal hydrogen compressor may comprise a metal hydride material for retaining and storing a concentrated volume of hydrogen gas, that material being capable of repeatedly absorbing and discharging gaseous hydrogen, the material comprising a mixture of water vapor absorbing particles, metal hydride particles and a noble metal in powder form. The material may further comprise a metallic powder selected from the group consisting of: Platinum black, Palladium black and Ruthenium black. In a second embodiment, a hydrogen compressor comprising an inlet for hydrogen gas fed at a low inlet pressure and an outlet for hydrogen gas at high pressure, therebetween at least two sets of connected units A, C and E and at least two sets of units serving the unit functions B, D and F said A through F being a first chamber in communication with said inlet through a one-way valve adapted to admit hydrogen gas into said first chamber at said low inlet pressure containing a first hydridable material having an adsorption pressure below said low inlet pressure at a first temperature, heat exchange means associated with said first chamber adapted to operate alternately to maintain said first chamber at or below said first temperature and to raise the temperature of said first chamber to a second temperature higher than said first temperature, a second chamber in communication with said first chamber through a one-way valve adapted to prevent flow of hydrogen from said second chamber to said first chamber and containing a second hydridable material forming a less stable hydride than said first hydridable material and having a plateau pressure at a temperature below said second temperature less than the plateau pressure of said first hydridable material at said second temperature, heat exchange means associated with said second chamber adapted to operate alternately to maintain said second chamber at a temperature lower than said second temperature and at a third temperature higher than said first temperature, a third chamber in communication with said second chamber through a one-way valve adapted to prevent flow of hydrogen from said third chamber to said second chamber and in communication with said outlet and containing a third hydridable material forming a less stable hydride the said second hydridable material having a plateau pressure at a temperature below said third temperature less than the plateau pressure of said second hydridable material at said third temperature, heat exchange means associated with said third chamber adapted to operate alternately to maintain said third chamber at a temperature lower than said third temperature and at a fourth temperature higher than said first temperature and control means for alternating the temperature capacity of heat exchange means B, D and F to maintain the lower of the two specified temperatures when hydrogen is being absorbed by the hydridable material in the associated chambers and at the higher of the two specified temperatures when hydrogen is present in and being desorbed from the hydridable material in the associated chambers, and said first chamber further including a hydrogen vent that is controlled by said control means for venting hydrogen gas from said first chamber at predetermined intervals and for a predetermined amount of time.